A viscose process has been known as a process for preparing fibrous cellulose (cellulose fibers). The term “viscose” refers to an intermediate obtained in one of techniques for producing rayon, a kind of regenerated cellulose. The term “viscose process” is a generic name for technologies to produce rayon through the intermediate viscose. The viscose is capable of giving articles in the form of not only a fiber (e.g., rayon), but also a film (e.g., cellophane). The viscose process was invented by E. J. Bevan et al. in 1892. Specifically, they found that cellulose can be regenerated by treating a cellulose compound with sodium hydroxide and carbon disulfide to give a viscous solution (viscose) and treating the viscous solution with an acid. The viscose process is schematically illustrated as follows. In the process, a natural cellulose fiber is chemically modified to dissociate intermolecular hydrogen bonds to once give a colloidal solution, and the colloidal solution is returned again to cellulose molecules to reaggregate or reassemble polymer molecules to thereby regenerate a fiber. The resulting cellulose fiber can have arbitrary length and shape. The viscose process can therefore give continuous fibers (monofilaments) that are unavailable in natural fibers.
The viscose process probably proceeds according to a reaction scheme as follows. Cellulose is treated with sodium hydroxide to form “alkali cellulose” in which a hydroxyl group at the 6-position of cellulose forms a sodium salt.[C6H7O2(OH)3]n+nNaOH→[C6H7O2(OH)2(ONa)]n+nH2O
The alkali cellulose is mixed with carbon disulfide and left stand to form sodium cellulose xanthate, which loses intermolecular hydrogen bonds, to thereby be dissolved and form a colloidal solution.[C6H7O2(OH)2(ONa)]n+nCS2→[C6H7O2(OH)2(OCSSNa)]n 
This is a yellow viscous liquid, just like the name implies (“xantho” in Greek refers to “yellow”) and is called “viscose.” The colloidal solution eventually becomes a reddish brown viscous colloid. The viscose is extruded from a small opening into diluted sulfuric acid for wet spinning. In this step, sodium cellulose xanthate is returned to cellulose and is regenerated as a cellulose fiber by the action of intermolecular hydrogen bonds.2[C6H7O2(OH)2(OCSSNa)]n+nH2SO4→2[C6H7O2(OH)3]n+2nCS2+nNa2SO4 
This is viscose rayon. The viscose, when extruded from a thin slit into a film, gives cellophane.
The so-called viscose process, as employing carbon disulfide for cellulose dissolution, disadvantageously suffers from fire risk caused by carbon disulfide and experiences the formation of large amounts of waste liquids (Disadvantage 1).
The viscose process also significantly disadvantageously suffers from reduction in molecular weight of the regenerated cellulose obtained. Specifically, cellulose such as pulp is immersed in a concentrated alkaline solution to form alkali cellulose, compressed and pulverized, and then fed to a ripening step. In the ripening step, the alkali cellulose is oxidized and disintegrated so as to have a decreased average degree of polymerization. The average degree of polymerization herein is generally decreased to the range of from about 300 to about 400. During this process, the distribution of degree of polymerization varies and is uniformized so that components with high degree of polymerization decrease, whereas components with low degree of polymerization accumulate (increase). This phenomenon remarkably occurs particularly in a cellulose source which includes a crystalline region and an amorphous region clearly distinguished from each other, in which oxidative cleavage of glucosidic bonds initially occurs in the amorphous region. Specifically, the disintegration herein is not a random disintegration. This causes distribution of the degree of polymerization (Disadvantage 2).
Independently, there has been known techniques of dissolving cellulose without such chemical modification as in the viscose process. In these techniques, cellulose itself is dissolved in a solvent to form a cellulose solution (dope) and the cellulose solution is spun to form fibers represented by Lyocell. Lyocell was developed and first manufactured for test production in 1988 by Courtaulds Fibres in UK. Lyocell is produced by dissolving a cellulose source in an aqueous N-methylmorpholine N-oxide solution to give a spinning solution called “dope,” and extruding the dope into a dilute solution of N-methylmorpholine N-oxide to give a Lyocell fiber.
The Lyocell fiber is produced without undergoing derivatization or another similar process, thereby less suffers from reduction in degree of polymerization of cellulose molecules, avoids Disadvantage 2, and has superior strengths. However, the Lyocell fiber undergoes a liquid crystal state during spinning, includes molecules being highly aligned longitudinally in the fiber, and often disadvantageously suffers from fibrillation in which the fiber tears longitudinally in the fiber axis direction (Disadvantage 3). The Lyocell fiber disadvantageously has rough and coarse touch and is generally hardly usable without further treatment. To prevent Lyocell fibrillation, various techniques have been proposed, but have failed to be fundamental improvements. In addition, the process to produce Lyocell employs N-methylmorpholine N-oxide (NMMO) and disadvantageously causes environmental issues as with Disadvantage 1.
Cellulose includes linear chains of glucose units, forms a semi-crystalline structure, and has a network structure of highly hydrophobic bonds. Cellulose is insoluble in water and most of regular solvents. Cellulose and derivatives thereof are derived from wood or cotton and are used as bioregenerative chemical materials. Exemplary chemical processes for obtaining cellulose derivatives include oxidation, decomposition, hydrolysis, esterification, alkylation, and copolymerization. Examples of such chemical processes include acetylation, acetylation-propionylation, acetylation-butyrylation, nitration, carboxymethylation, ethylation, and hydroxyethylation. Non-derivatized cellulose is soluble only in limited solvents such as alkylpyrrolidone halides.
There have recently been proposed techniques of using ionic liquids for cellulose dissolution. Non Patent Literature (NPL) 1 (Electrochemical Society Proceedings Volume 2002-19, 155-164) reports a technique of employing, as an ionic liquid, a salt with an 1-alkyl-3-methylimidazole as a cation, and dissolving cellulose in the ionic liquid. More specifically, the technique gives a 10% cellulose solution by heating the 1-alkyl-3-methylimidazole chloride and cellulose at 100° C. The solution exhibits anisotropic optical properties under observation using crossed Nicols, indicating that the solution is in a liquid crystalline phase. The solution, as being in a liquid crystalline phase, causes a disadvantage as with Disadvantage 3. The ionic liquids are expensive. Of the ionic liquids, those employing chlorides disadvantageously cause corrosion of production facilities, because the chlorides contaminate the recovered solution and collected cellulose. In addition, when cellulose is esterified after dissolving the same in an ionic liquid, water is inevitably by-produced, and the separation of water from the ionic liquid requires huge energy, resulting in a heavy environmental load (Disadvantage 4). Furthermore, the resulting cellulose obtained by heating disadvantageously undergoes a color change (Disadvantage 5).
Patent Literature (PTL) 1 (Japanese Unexamined Patent Application Publication (JP-A) No. 2008-50595) discloses a solvent for cellulose dissolution, which solvent includes an imidazolium carboxylate or another ionic liquid and a specific amount of water. Even this solvent, however, requires heating for cellulose dissolution.
There have also been known techniques for dissolving cellulose in ionic liquids without heating. NPL 2 (Green Chem., 2010, 12, 1274-1280) reports that an 1-ethyl-3-methylimidazolium alkyl phosphate, when used as an ionic liquid, dissolves cellulose therein and gives a 10 percent by weight cellulose solution without cellulose decomposition by stirring at 45° C. for 30 minutes. The dissolution is accelerated by heating the mixture to 70° C. This ionic liquid is neither volatilized nor thermally decomposed until it reaches 250° C., and can resist long-term heating at 100° C.
In addition, there have been proposed techniques of dissolving cellulose in ionic liquids and subjecting the resulting cellulose solutions to derivatization such as acetylation. Typically, PTL 2 (U.S. Pat. No. 8,166,267) discloses a technique of dissolving cellulose in an ionic liquid and subjecting the resulting cellulose solution to acetylation. These techniques, however, still fail to solve Disadvantage 4.
Independently, a core-shell structured cellulose ester-cellulose composite has received attention as a functional polymeric material. Exemplary known techniques for producing a core-shell structured cellulose ester-cellulose composite include a technique of preparing a solid of cellulose acetate, and hydrolyzing the solid of cellulose acetate in the presence typically of a sulfuric acid catalyst. This technique hydrolyzes the surface of the cellulose ester shaped article and gives a composite as a core-shell structured cellulose ester-cellulose composite including cellulose as the surface layer (shell) and the cellulose ester as the core.
There has been also disclosed a technique of heterogeneous acetylation so as to produce cellulose esters, particularly cellulose acetate. This technique includes esterifying beaten pulp or another cellulose source in the presence of an esterification catalyst to produce cellulose acetate having a desired degree of substitution without obtaining primary cellulose. The technique may give a composite including a cellulose ester as the surface (shell) and cellulose as the core. The technique, however, fails to give a core-shell structured cellulose-cellulose ester composite typically in the form of a monofilament because the technique can employ only a solid cellulose source (e.g., beaten pulp) processed from naturally occurring materials such as wood.
In addition, customary techniques for producing a cellulose ester from cellulose disadvantageously fail to remove sulfuric esters completely from the resulting cellulose ester. Exemplary known cellulose sources include wood pulp and cotton linters. Cellulose in these materials has a crystal structure called “cellulose I.” Techniques of esterifying cellulose having the crystal structure cellulose I are well known. Assume that cellulose having the crystal structure cellulose I is acetylated, where acetylation is most easily performed among esterification processes. Even in this case, acetylation of cellulose in the crystalline region requires as an acetylating agent not a regular acetylating agent, but acetic sulfuric anhydride obtained through reaction between catalyst sulfuric acid and acetic anhydride. For this reason, even acetylation, through which esterification is easily performed, essentially requires such sulfuric acid catalyst for the acetylation of cellulose in the crystalline region. Still more, propionylation reaction according to the customary technique cannot proceed by using large-molecule propionic anhydride alone, but further requires acetic sulfuric anhydride, and results in a cellulose ester of mixed fatty acids, i.e., cellulose acetate propionate.
The use of such sulfuric acid catalyst in the processes for producing a cellulose ester from cellulose is known to cause sulfuric esterification (sulfation) of part of the cellulose ester molecules. The resulting sulfuric ester is known to act as a hydrolysis catalyst during the storage of the cellulose ester. To prevent this, various attempts have been made for the removal of the sulfuric ester (combined sulfuric acid) in cellulose ester production. Such customary techniques, however, disadvantageously fail to remove the sulfuric ester completely from the product cellulose ester (Disadvantage 6). NPL 3 (Kogyo Kagaku Zasshi (in Japanese; Journal of the Chemistry Society of Japan), 1941, 44, 16-22) describes that the hydrolysis of sulfuric ester groups in cellulose acetate to a content of significantly lower than 0.1 percent by weight is remarkably difficult or impossible.